An armature of this type comprises superposed layers of threads, particularly of carbon, glass or boron, generally transversely connected by transverse sewing or through threads.
These armatures are made in two steps. During the first step, so-called weaving, there is produced a superposition of oriented layers, each layer being formed by depositing threads constituting several thousands of threads with the help of guides reciprocating on a frame provided with hooking devices called pins disposed on its periphery.
In the second step, so-called connecting, transverse threads are emplaced by penetration of the stack of layers by one or several needles.
In the patent FR-A-2 718 757 of the applicant, there is described a process and machine for the production of an armature of the above type.
In this patent, there is described a weaving device constituted by a frame surrounding a mass of flexible material and provided with hooking pins implanted about the periphery and one side of the frame.
These pins are simple straight pins disposed parallel to the plane of the frame and at equal distances from each other. The weaving consists, for each layer, in depositing sections of threads parallel to a direction while stretching a thread between two pins on opposite sides of the frame.
The sections of threads being deposited one by one and successively, it will be understood that a substantial amount of time is needed to constitute the desired stack, which can comprise about ten superposed layers. Moreover, it is very delicate to obtain a regular superposition of the layers, particularly at the height of the pins, because the passage of the thread about them while changing direction is not carried out in a superposed manner and with the same spacing. There follow problems of irregularity throughout the mass of the armature which does not have a precise and constant thickness in particular at its periphery.
These problems are not completely corrected by the flattening which is applied after laying down of the successive layers.
Thus, this flattening takes place by a presser plate on the stack of layers, from a single side of the latter because the other side is masked by said mass of flexible material filling the interior of the weaving frame.
It is not possible to maintain such a flattening during the ultimate binding operation because it is necessary to disengage the stack of layers to permit the binding needles to penetrate the fibrous mat thus constituted so as to connect with one thread and without knotting, said layers, the flexible material disposed on the fibrous mat serving to retain transverse binding threads.
The presence of the flexible material on one of the surfaces of the mat in the course of stitching thus ensures a certain tension of the transverse threads contributing with the preliminary flattening to obtaining armatures having a better compactness.
However, optimum compactness is not obtained, because the above flattening is not maintained during stitching and because of resilient reaction the different superposed layers have the tendency, once free from pressing, to space apart, this tendency not being completely counterbalanced by the tension exerted by the transverse threads.
Moreover, the technique set forth in FR-A-2 718 757 is not applicable to binding with knotting, which implies free access to two opposite surfaces of the fibrous mat.
In FR-A-2 730 247, there is disclosed a process for the production of a three-dimensional cloth constituted by superposed fibrous layers interconnected transversely by a connection with knotting, more particularly by a chain stitch.
To this end, there is provided an embodiment for using a weaving device analogous to that of FR-A-2 718 757, with the difference that the interior of the frame is totally disengaged, thereby permitting access to the two opposite surfaces of the fibrous mat.
This weaving device of course has the same drawbacks as to homogeneity and compactness of the armatures thus produced.